1. Field of the Invention
The present invention relates to materials for organic electroluminescence devices and organic electroluminescence devices using the materials and, more particularly to materials for organic electroluminescence devices which realize electroluminescence devices exhibiting a high emitting efficiency, causing little pixel defects, exhibiting a high heat resistance and having a long lifetime.
2. Description of the Prior Art
An organic electroluminescence device (organic ELdevice) is a spontaneous emission device which utilizes the phenomenon of fluorescence which occurs by the energy of recombination between holes injected from an anode and electrons injected from a cathode by application of electric field. C. W. Tang of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, 51, p. 913, 1987) reported a laminated organic EL device which is driven under low electric voltage. Since that time, many studies have been made on organic EL devices using organic materials as the constituting materials. Tang et al. used a light emitting layer of tris(8-quinolinolato)aluminum and a hole transport layer of a triphenyldiamine derivative. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, that the efficiency of forming excitons by recombination can be increased by blocking electrons injected from the cathode, and that excitons formed can be confined in the light emitting layer. As the structure of organic EL devices, a two-layered structure having a hole transport (injection) layer and an electron transport/light emitting layer and a three-layered structure having a hole transport (injection) layer, a light emitting layer and an electron transport (injection) layer are well known. To increase the efficiency of recombination of injected holes and electrons in laminated devices, the structure of the device and the process for forming the device have been studied.
As the light emitting material of organic EL devices, chelate complexes such as tris(8-quinolinolato)aluminum, coumarin derivatives, tetraphenylbutadiene derivatives, distyrylarylene derivatives and oxadiazole derivatives are known. It is reported that these light emitting materials emit visible light ranging from blue to red and the development of color display devices is expected (for example, Patent Documents 1-3).
It is recently proposed to use a light emitting layer of a phosphorescent material in addition to a light emitting layer of a fluorescent material (for example, Non-Patent Documents 1 and 2). A high efficiency of light emission is achieved by utilizing the excited singlet state and the excited triplet state of the organic phosphorescent material in the light emitting layer. It is considered that the singlet exiton and the triplet exciton are formed in a proportion of 1:3 due to the difference in the spin multiplicity when electrons and holes are recombined in an organic EL device. Therefore, it is expected that an efficiency of light emission 3 to 4 times as great as that of a device utilizing only the fluorescent material can be achieved by utilizing a phosphorescent light emitting material.
To prevent the excited triplet state or the triplet exciton from quenching, the organic EL devices described above are made into a laminate structure having an anode, a hole transport layer, an organic light emitting layer, an electron transport layer (a hole blocking layer), an electron transport layer and a cathode in this order, while using an organic light emitting layer made of a host compound and a phosphorescent compound (for example, Patent Documents 4-9). In these Patent Documents, host materials having a dibenzofuran structure or a dibenzothiophene structure are described. However, there is nothing about their advantage in the device performance as compared with a host material having a carbazole structure.
Patent Documents 10 and 11 disclose compounds prepared by bonding a carbazole structure to dibenzo compounds. The proposed compounds are used in a host material for a blue phosphorescent device in examples thereof. The compounds of the present invention are not taught and the effect is uncertain.
Patent Documents 12 to 14 disclose compounds prepared by bonding an anthracene structure to dibenzo compounds. However, the energy level of the excited triplet state of the proposed compounds is small because of the anthracene structure. Therefore, a blue phosphorescent material does not emit light even if any of the proposed compounds is used as a host material for the light emitting layer.
Patent Document 15 discloses dibenzofuran compounds essentially having at least two polymerizable functional groups. If the host material of a phosphorescent device has in its molecule a polymerizable group such as propenylene group, vinylene group and 4-propyl-2-pentenylene group which is formed by introducing an unsaturated bond such as double bond and triple bond, radicals are propagatedly generated in the device to adversely affect the emitting efficiency and lifetime.    [Patent Document 1] JP 8-239655A    [Patent Document 2] JP 7-138561A    [Patent Document 3] JP 3-200889A    [Patent Document 4] WO 05/101912    [Patent Document 5] JP 5-109485A    [Patent Document 6] JP 2004-002351A    [Patent Document 7] WO 04/096945    [Patent Document 8] JP 2002-308837A    [Patent Document 9] WO 2005-113531    [Patent Document 10] JP 2005-112765A    [Patent Document 11] WO 2006-114966    [Patent Document 12] JP 2005-314239A    [Patent Document 13] JP 2007-77094A    [Patent Document 14] JP 2007-63501A    [Patent Document 15] JP 2007-110097A    [Non-Patent Document 1] D. F. O'Brien and M. A. Baldo et al. “Improved energy transferring electrophosphorescent devices” Applied Physics letters Vol. 74 No. 3, pp 442-444, Jan. 18, 1999    [Non-Patent Document 2] M. A. Baldo et al. “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” Applied Physics letters Vol. 7.5 No. 1, pp-4-6, Jul. 5, 1999